1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium mounted in various magnetic recording apparatuses, and in particular to improvements of an initial growth layer in a magnetic recording layer and in the dispersion angle of a C-axis of a magnetic recording layer.
2. Description of the Related Art
In recent years, rather than focusing on conventional longitudinal magnetic recording method, a vertical magnetic recording method has been attracting attention as a technology to increase magnetic recording density. A medium based on the vertical magnetic recording method (hereinafter referred to as a “perpendicular magnetic recording medium”) is composed primarily of a magnetic recording layer made of a hard magnetic material, a under layer that orients the magnetic recording layer in a target direction, and a protective film that protects the surface of the magnetic recording layer. The perpendicular magnetic recording medium may have a backing layer composed of a soft magnetic material to concentrate magnetic fluxes generated by a magnetic head used for recording on the magnetic recording layer. Typically, the perpendicular magnetic recording medium having no soft magnetic backing layer is referred to as a “single-layer perpendicular magnetic recording medium,” and the perpendicular magnetic recording medium having a soft magnetic backing layer is referred to as a “double-layer perpendicular magnetic recording medium.” Various vertical magnetic recording media have been examined, but it is desired not only to increase the magnetic recording density but also achieve further improvement in performance, such as an increase in an original signal or a decrease in noise.
A factor in degraded performance of the perpendicular magnetic recording medium is an increase in the dispersion angle of a C-axis of the magnetic recording layer. In the vertical recording medium, an easy axis of magnetization of the magnetic recording layer must be oriented perpendicularly to a surface of the medium. However, such orientation alone is insufficient. An increase in the dispersion angle of the C-axis of the magnetic recording layer reduces an amount of magnetic fluxes in the vertical direction and thus lessens the signal output. Further, the amount of magnetic fluxes leaking obliquely from adjacent bits increases, thereby increasing noise. That is, an increase in the dispersion of the C-axis of the magnetic recording layer reduces S/N, which is indicative of a ratio of the signal output from the perpendicular magnetic recording medium to noise occurring therein. Accordingly, to improve the performance of the perpendicular magnetic recording medium, the dispersion of the C-axis of the magnetic recording layer must be minimized.
Another factor in degraded performance of the perpendicular magnetic recording medium is a creation of an initial growth layer in the magnetic recording layer. The magnetic recording layer in the conventional perpendicular magnetic recording medium includes an initial growth layer having a thickness of approximately several nm, and having magnetized components in the in-plane directions. This initial growth layer causes noise. Furthermore, the magnetic recording layer desirably has a small film thickness to enable proper recording the initial growth layer, a decrease in film thickness allows the initial layer to occupy a relatively larger part of the film thickness, thereby reducing the S/N, which hinders a reduction of the film thickness of the magnetic recording layer. Accordingly, to improve the performance of the perpendicular magnetic recording medium, it is necessary not only to reduce the dispersion of the C-axis of the magnetic recording layer, but also to reduce an amount of creation of the initial growth layer in the magnetic recording layer.